The starting phase for all stars, including our Sun, begins when a dense region in a nebula begins to shrink and warm up. This is usually the result of one of several events that may occur to initiate the gravitational collapse of a molecular cloud. The means by which this occurs include galactic collisions or a devastating nearby supernova explosion sending ruptured matter into the clouds at very high speeds. Each of these stellar maternity wards can form anything from a few dozen to thousands of stars.
To form a star like our Sun, which is 864,400 miles (1,391,000 kilometres) across, it would take a collection of gas and dust a hundred times the size of our solar system. This is just the beginning. Once such a large amount of gas and dust huddle together, they form what we call a protostar. An object is considered a protostar for as long as material is still falling inward. For our Sun, and stars of the same mass, the protostar phase would have ended after approximately 100,000 years. After this, the protostar stops growing and the disk of material surrounding it is destroyed by radiation.
If the protostar was unsuccessful in acquiring enough mass, a brown dwarf will come into shape. These poor little guys are substellar objects that are unable to sustain hydrogen fusion reactions in their cores, due to their insufficient mass. Main sequence stars have no issue with this, to the envy of brown dwarfs. Putting it simply, a brown dwarf is too big to be called a planet, and too small to be called a star. Until 1995, they were only a theoretical concept. It is now thought, however, that there is a brown dwarf for every six stars.
If the star is big enough to fuse hydrogen atoms into helium, it will enter the phase that our Sun is in, called the main sequence phase. A star will enjoy most of its life in the main sequence phase. At this point nuclear fusion is turning hydrogen into helium. The star is only stable because the light pressure of this energy balances out the star’s gravitational collapse.
Approximately nine out of ten stars in the universe are main sequence stars. These stars can range from around a tenth of the mass of our Sun all the way up to 200 times as massive, and how long a star will stay in the main sequence phase depends on its size. A star with higher mass might have more material to play around with, but it will burn faster due to higher core temperatures caused by greater gravitational forces. A star the size of our Sun will spend about 10 billion years in this phase, but a star 10 times the size of our own will stick around for only 20 million years.
After the main sequence phase, the star will become a red giant. A red giant is a dying star in one of the last stages of stellar evolution. In a few billion years’ time, our Sun will die and expand, gobbling up the inner planets, and maybe even the Earth (don’t worry; we’ll have died out a few billion years earlier. If we do manage to survive for another billion years, the temperature of the Earth’s surface will become far too hot for us humans.)
After stars stop converting hydrogen into helium via nuclear fusion, gravity will take over. It’s all downhill from here, I’m afraid. Red giant stars reach sizes of 62 million to 621 million miles in diameter (100 million to 1 billion kilometres), 100 to 1,000 times the size of the sun today. The energy of the star is spread out across a larger area, like the pixels when one expands a raster graphic. Because of this, the star actually becomes cooler reaching only a little more than half the heat of the Sun. The temperature change causes stars to shine more towards the red part of the spectrum; it is this that gives a red giant its name.
Where a star goes from this point depends on its size. Let’s first go with the less violent option. Smaller stars, up to around eight times the mass of our sun, can become a white dwarf. These old stellar remnants are incredibly dense. A teaspoon of their matter would weigh as much on Earth as an elephant – that’s 5.5 tons in one incredibly strong teaspoon. A white dwarf’s radius is just .01 times that of our Sun, but the mass is about the same. Estimating how long a white dwarf has been cooling helps astronomers increase their understanding of how old the universe really is.
After an unimaginable amount of time – tens or even hundreds of billions of years – a white dwarf will cool until it becomes a black dwarf, which are invisible because they are emitting at the same temperature as the microwave background. Because of the age of the universe and what we know about its oldest stars, there are no known black dwarfs.
Alternatively, a star with at least eight solar masses will have a much more violent, yet much more beautiful, death. Massive stars can create a supernova when they run out of fuel. To them, it’s better to go out with a bang than to fade away. When supernovae explode, they fling their guts into space at speeds of 9,000 to 25,000 miles per second.
These blasts produce much of the material in the universe including some heavy elements such as iron, which help to make up both ourselves and our planet, so all of us carry the remnants of these explosions in our bodies. As Neil deGrasse Tyson puts it, “It is quite literally true that we are stardust.” The cycle starts all over again, with a new generation of stars, and new stars are born from the stardust left behind in the same way.
That doesn’t mean it’s the end of the road for what remains of the star. After the supernova explosion, the star’s core is left behind in the form of either a black hole or a neutron star, both of which are incredibly destructive and violently beautiful. Neutron stars are hard to find and are very mysterious objects. They may only be about the size of a city, but don’t let that fool you, these objects are not to be messed with. They are extremely dense: if one takes the mass of our sun, doubles it, and then shrinks it down to the size of Los Angeles, that’s roughly how dense a neutron star is. A cubic metre of a neutron star would weigh just less than 400 billion tonnes. All of that density makes their surface gravity truly immense.
Alternatively, what’s left after the supernova can become a black hole. Black holes literally pull the space around them. They need to have a massive amount of mass in an incredibly small space to have the required gravity to pull in light. To put this into perspective, to make a black hole out of the Earth the entire planet would need to be squeezed down to the size of a pea! These mysterious and frightening objects can slow down time and rip you apart and nothing can escape the grasp of a black hole when it reaches its event horizon. Any matter that enters its path is never seen again. They’re the playground bully of the universe, but unlike playground bullies, we might depend on them to live. Some researchers think black holes actually help create the elements because they break down matter into subatomic particles.
These particles make up you and I, and everything around us. We owe the stars our lives. Whether it’s big or small, young or old, you can’t argue that stars are some of the most beautiful and poetic objects in all of creation. Next time you look up at the stars, remember, this is how they were all created and how they will die.